Bootloaders come in all shapes and sizes. The kind of characteristics you want from a bootloader are that they be simple and customizable with lots of sample configurations for common development boards and devices. The following table shows a number of them that are in general use:

We are going to focus on U-Boot because it supports a good number of processor architectures and a large number of individual boards and devices. It has been around for a long time and has a good community for support.

It may be that you received a bootloader along with your SoC or board. As always, take a good look at what you have and ask questions about where you can get the source code from, what the update policy is, how they will support you if you want to make changes, and so on. You may want to consider abandoning the vendor-supplied loader and use the current version of an open source bootloader instead.